MacBook RAM Question

I currently own a White 2.1 GHz MacBook, and I've been noticing that PhotoShop and Aperture are lagging quite a bit on 1GB of RAM. I was wondering, how much should I upgrade to get them to run at decent speeds? Would 2GB cut it, or am I better off just going all the way to 4GB? Are there any good deals going on (preferably in a local store for purchase rather than online) currently?

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